Gasifier

ABSTRACT

An apparatus and process for converting carbonaceous or other material with calorific value, wood, plastic, sewage, etc into high quality gas preferably to fuel a reciprocating gas engine for the generation of electricity. Wet fuel is delivered via conveyer  13  to fuel hopper  14.  From the hopper the fuel is fed into the dryer  15  by a screw feeder The dried fuel then is checked for size via a trommel  16  where the correctly sized fuel passes through and the oversized fuel goes onto the reject conveyer  22  where it is delivered for shredding. The correct sized dry fuel is transported via a conveyer  17.  The fuel is then fed via a feed system, to avoid the ingress of air, into the gasifier  19.  The gas is cooled and cleaned in the gas quench unit  20.  The gas is then compressed and stored. The char is quenched in a water trough then fed by a screw conveyer to a dryer  29  then stored in a hopper  30.  Oils and tars that are carried over with the gas are removed by an extraction unit  27  and stored in storage vessel  28.  From storage the oils are used as a fuel along with the char via burners  1  to fuel the char reburner  2.  The combustion air used in the char reburner is taken from the dryer. This air is dried in  25  by the forced draft fan  26  then sent to the combusters 1.  Any slag that is produced in the char reburner is ejected as a vitrified slag into a trough  32  where it is removed for further processing. The hot gases from the char re-burner are treated by Nox suppression system  3.  The temperature is then controlled at  4  before heating the gasifier  5.  Having heated the gasifier the hot gases are guided via ducts  6  to a further stage of temperature control  7.  From there they heat the dryer before being guided via duct  9  to a bag filter  10  that collects any dust allowing the clean exhaust to pass to the chimney  12  via an induced draft fan  11.

The present invention relates to an apparatus and process for thegasification of any carbonaceous or other material of useable calorificvalue to produce a high quality gas preferably to fuel a reciprocatinggas engine for the generation of electricity Sources of traditionalfossil fuels and hydrocarbons have a finite life and there is evergrowing pressure from environmental groups, as well as governmentauthorities, to clean up the planet. There is also internationalpressure to suppress noxious emissions that are causing climate change.Waste to energy systems are known, but mainly rely upon incinerationwith high capital cost and production of large quantities of dirty ash,and are increasingly becoming unacceptable.

The present invention provides an efficient solution, at a relativelymodest capital cost. The system provides significant improvements towell known, long established technology, with the advantage of allowinga modular and adaptable system to be custom built to suit thecomposition and quantity of the waste supply. The design has lowmaintenance costs and beats by a substantial margin all emission targetsset by international and domestic treaties and agreements. A wide rangeof fuels can be processed including, but not limited to, forestry waste,municipal waste after removal of metals, food waste including factoryprocessing waste, sewage, animal waste and rubber tyres.

The process is one of continuous flow. The waste is dried and metals, ifpresent, are extracted. Any plastics, glass etc can also be removedalthough this is not essential. The waste is then graded, with theoversize material being shredded and re-introduced. The fuel thusproduced is then injected into an unique anaerobic gasifier and gasifiedat about 800° C. The gas is cooled and filtered to remove contaminantsbefore being fed into gas engines or gas turbines for power generation.The solid residues from the gasifier together with any oils and tars arethen introduced into a secondary gasifier to produce further gas andheat for use in the cycle. The minimal residues are converted to aninert vitrified slag for use in the construction industry. Hence thefull process has no unusable residues.

The gasifier consists of a substantially horizontal, cylindrical reactorwhich rotates slowly within a refractory lined furnace vessel. The wastematerial is indirectly heated in an oxygen free atmosphere. The gasproduced, after cooling and cleaning, can be used to generate “green”electricity via a gas engine or gas turbine. Thermal energy producedalso has profitable uses.

The main feature of the design is the provision of an innovatoryinternal vane arrangement which allows homogeneous distribution of thefeed material over a large area of the retort. This exposes it quicklyto the heat without the need for rapid tumbling and agitation that isused in competing processes. Furthermore “cold spots” are avoided, thusincreasing the plant's ability to produce gas of a consistently goodquality.

As a result of the internal configuration, the design is also animprovement on the conventional rotating reactor design because itpermits the use of a more compact unit. A slight increase in diameterallows the use of a shorter vessel. Thermal analysis confirms that thesystem provides the correct rate of heating needed to generate goodquality gas. The refractory lining acts as a good heat sink and givesthe required temperature stability.

The design is robust and has the advantage that, whilst innovative, itdraws on proven engineering principles and also avoids the problemsassociated with other systems that incorporate high speed moving parts.

By way of example only, a specific embodiment of the present inventionwill now be described with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic view of a gasification plant in accordance withthe present invention.

FIG. 2 is a longitudinal section of the gasifier.

FIG. 3 is a cross section of the gasifier kiln.

FIG. 4 is a diagrammatic view of the secondary gasifier showing theprincipal flow patterns. The dotted lines are not structural. They showflow boundaries.

Referring to FIG. 1, wet fuel is delivered via conveyer 13 to fuelhopper 14. From the hopper the fuel is fed into the dryer 15 by a screwfeeder. The fuel rolls around the dryer and is heated to evaporate offthe moisture. The drying process also serves to sterilize the fuel feed.The dried fuel is then graded for size via a trommel 16 where thecorrectly sized fuel passes through and the oversized fuel goes onto thereject conveyer 22 where it is delivered to a reject skip 23 for furtherprocessing.

The correct sized dry fuel is transported via a conveyer 17, along withthe fuel that did not require drying. Both fuels are stored in the dryfuel hopper 18.

The shredding of oversized dry fuel is carried out by selected equipmentsuitable for the material to be shredded (eg. tyres, dry industrialwaste). On completion the material is carried by a separate conveyor anddropped onto conveyor 17. Oils and other liquid fuels are stored intanks and pumped into the gasifier 19 or bio oil storage tank 28 to fuela secondary gasifier depending on the fuel combinations being processedand the respective heat balance.

The fuel is then fed via an elaborate purged feed system, to avoid theingress of air, into the gasifier 19 where it is heated to separate thegas from the solid char. The gas is cooled in the gas quench unit 20where it is also cleaned. The gas is then compressed and stored in a gasstorage unit. Then it is used to fuel a gas engine to generateelectricity.

The char is quenched in a water trough then fed by a screw conveyer to adryer 29 then stored in a hopper 30. Oils and tars that are carried overwith the gas are removed by an extraction unit 27 and stored in storagevessel 28. From storage the oils are used as a fuel along with the charvia burners 1 to fuel the secondary gasifier 2. The combustion air usedin the secondary gasifier is taken from the dryer. This air is dried in25 by the forced draft fan 26 then sent to the combusters 1. Any slagthat is produced in the secondary gasifier pours out from the secondarygasifier as a vitrified slag into a trough 32 where it is removed intoskips 33.

The hot gases from the secondary gasifier are treated to reduce NOx byNOx suppression system 3. The temperature is then controlled at 4 beforeheating the gasifier 5. Having heated the gasifier the hot gases areguided via ducts 6 to a further stage of temperature control 7. Fromthere they heat the dryer before being guided via duct 9 to a filter 10that collects any dust allowing the clean exhaust to pas to the chimney12 via an Induced draft fan 11.

The preferred plant equipment is further described in the followingsections.

The process takes any carbonaceous material, wood, plastic etc with asize less than 16mm. Gas quality is initially determined by the overalltemperature and subsequently by the gas temperature and gas residencetime in the gasifier. By a process of rapid heating in the absence ofair in the rotary kiln gasifier, the sorted waste is gasified to producesignificant quantities of gas fuel. To maintain the safety and integrityof the gasifier a pressure release system is needed to enable the swiftrelease of gas. As the gas will be at a high temperature its releaseinto the atmosphere will cause it to self ignite. Thus this emergencyrelease will be directed through a low-pressure flare immediately abovethe gasifier. A nitrogen purge system ensures safe operation duringstart up and shutdown.

Wet fuel is transported, by conveyor, to a storage hopper that has avolumetric capacity for 3 hours at maximum output. The fuel is fed bygravity into a screw feeder then into the dryer. This allows for controlof the feed rate into the dryer. This unit will improve the energyefficiency of the overall process by using waste heat to drive offmoisture.

In the dryer, the fuel will be dried to water content of less than 8%,dependent on the inlet water burden, though if this rises above 40% thenthe “dried” fuel moisture content may rise above 8% whilst the systemstabilizes.

The dryer is a rotating kiln type dryer with internal fins to increasethe heating surface area and to keep the fuel moving. Dryer temperatureis controlled to maintain a fuel temperature of around 125° C. to 140°C. so as to minimize premature pyrolysis of the waste. The dryer isdesigned for fuel temperatures of 240° C. Temperature control is bydilution air added to the hot exhaust gases from the gasifier. These arethermally controlled to supply the required temperature to the dryer.

The dryer runs at a constant speed and the control variables are wetfuel feed rate and heating temperature. The fuel dwell time in the dryeris controlled by the incline of the dryer and is pre-set to 20 minutes.The dryer exhaust gases then pass through a ceramic fabric filter forremoval of particulates. An Induced air fan draws the exhaust gasesthrough the dryer and filter. The end of the dryer has a trommel thatrejects the fuel of a size greater than ⅝ of an inch. The correctlysized fuel travels to the dry fuel hopper and the rejects fall into askip.

The wet air from the dryer is drawn off. This air is then dried throughcooling to provide dry air to the forced draft fan. The water collectedis used as part of the secondary gasifier de NOx system

The dry fuel is stored in the dry fuel hopper above the gasifier. It hasa 3-hour supply of fuel at maximum load. By gravity the fuel falls intoone of 2 sets of hydraulically actuated ram loaders, where waste ischarged incrementally to the gasifier. Each loader is phased, onefilling whilst the other is feeding the gasifier. A purpose designedfeeding mechanism has been provided which ensures a positive sealbetween the gasifier and atmosphere, purge entrained air from the fueland positively feed the fuel to the gasifier.

Based on the principles previously outlined, the gasifier is a rotarykiln consisting of a rotating, slightly inclined metal shell or tube,which progressively transports the fuel along its length, and iscontained within a refractory lined static metal shell. The exhaust gasfrom the secondary gasifier external to the kiln heats the tube.

A quench system cools the gas, and a gas clean up plant then ensuresthat the gas is suitably cleaned of contamination. An effluent plantneutralizes the effluent streams from the gas clean up system. Thefunction of the gas clean up plant is to remove the contaminants fromthe gas stream. Cleaning is required to prevent contaminants fromcausing a problem within the downstream equipment such as rapid cloggingof filters and corrosion of gas engine internals etc.

The particulates are removed by physical separation, whilst the halidesand sulphurous compounds are removed by chemical reaction, The plantalso includes a polishing filter to remove trace compounds includingdioxins, furans and heavy metals.

Gas and minimal liquid products exit the gasifier in the gaseous phaseand pass through a quenching spray that reduces the gas temperature andsaturates the gas to its adiabatic condition. This allows condensationof the oils and tars and performs a degree of solids removal.

Owing to the expected levels of condensable tars and oils it was decidedthat a wet quench would be more prudent. The condensates of the volatilehydrocarbons are collected and removed regularly to the bio oil storagetank. Operational experience will determine the actual frequency.

The design requires a use of chemicals based on the normal expectedlevels of contaminants. This is very much dependent on the compositionof the waste.

A gas compressor is situated before the carbon filters to ensure thatthe gas is drawn through the gas clean-up plant and is of sufficientpressure to pass through the carbon filter and then feed the gas engineand also to achieve the gas storage compressor's inlet pressure.

Liquors are collected from the plant and delivered into a liquor holdingtank. These will include the blow down from the char quench system andspent materials from the scrubbing stage. The effluent will includechlorine, fluorine, sulphur and hydrogen sulphide contaminants, whichare oxidised at stabilized pH levels by the addition of sodium hydroxideand sodium hypochlorite. The liquors are then injected into thesecondary gasifier.

Chlorine compounds, a precursor to the formation of dioxins and furans,are expected to be present in the gas. However, their formation in thegasification process will be minimal owing to the absence of significantquantities of oxygen Nevertheless, provision is made in the gas clean upplant to remove chlorine compounds.

The transition time of the fuel from initial entry to ash removal isdetermined by the angle of inclination, the speed of rotation beingpre-set. The angle of inclination can be adjusted manually. The gasifieris designed to heat up the fuel as quickly as possible to the pyrolysistemperatures in order to minimize carbon in the ash. The temperature ofthe gas will initially be determined by the temperature of the fuel whenthe gas is given off, and subsequently by heat gained by the gas fromthe shell and from the ash being tumbled by the gasifier.

Solid ash or char residue from the gasifier is deposited into awater-cooled receiver to reduce the temperature. The char is then groundand transported via a screw conveyor into the char storage hopper. Fromhere the char will be collected via bottom silo empting rotary valvesthrough gravity and carried in the primary combustion air into a charcombustor.

The char, carbon and ash, from the gasifier is used as the primary fuelin the secondary gasifier, together with the tar and oil collectedduring the gas quench process. Added to this will be any concentratedeffluent and the dust from the filters. The non-combustibles arevitrified in the secondary gasifier to produce a slag. This vitrifiedslag is used in the construction industry.

A gas storage facility is also provided to smooth out variations in gasquality caused by changes in the waste stream. The storage tanks canalso supply gas for short periods when the gasifier is not producinggas, for example during start-ups.

The gas is preferably fed into a gas engine, which drives an alternatorto generate electrical power for export into the local grid network. Theexhaust heat from the gas engines is added to the system to support theprocess.

The secondary gasifier is designed to form a molten slag from the ashproducts encapturing pollutants and to produce a vitrified slag.

The refractory-lined vessel is fired by multiple fuel burners that canoperate on gas, fuel oil, Including the bio oils and tars and the charfrom the gasifier.

As the fuel is injected into the secondary gasifier it is gasified andas the gas burns progressively as it travels down through the secondarygasifier it is aerodynamically forced to rotate at high speed.

The secondary gasifier has a unique flow made up of two aerodynamicspirals, one inside the other. The outer spiral rotates downwardstowards a vortex collector cone, and as the flames spin down the conethe rotation inverts into a vortex. This causes the remaining ash toeject into the molten pool that collects and finally drains into a watertrough by gravity, leaving cleaned gases to spiral through the ultrahigh temperature central axis.

Centrifugal force retains the heavier un-gasified fuel with ash productsand other matter around the periphery of the cylindrical chamber thusproviding a longer gasification/burn time and thus reducing theemissions. This also cools the walls, which run at less than 900° C. forlong refractory life. As the gases burn out and become rarefied at veryhigh temperature they rotate towards the central outlet. Circling theflames back on themselves and blending layer upon layer achievesregenerative combustion, which increases the temperature on residenceperiod to several times that of conventional burners.

Immediately after the secondary gasifier, a water and urea injectionsystem reduces the temperature and NOx levels. This system consists of aurea concentrate holding tank and a mixing tank where the water from thecombustion air dryer is delivered. The mixing tank is controlled to givethe correct consistency of urea and water to minimize the NOx levels.

Whilst the invention has been described in detail in terms of a specificembodiment thereof, it will be apparent that various changes andmodifications can be made by one skilled in the art without departingfrom the spirit and scope thereof.

1. An apparatus and process for the treatment of material with asignificant usable convertible calorific value comprising an externallyheated rotary, approximately horizontal, un-pressurised internalcylindrical vessel within a static outer vessel with a feed mechanismfor ingress of suitably sized fuel designed to ensure anaerobicconditions within the said inner vessel.
 2. An apparatus as claimed inclaim 1, wherein the longitudinal axes of the vessels are inclined at anangle in excess of 1% to the horizontal.
 3. An apparatus as claimed inclaim 2 in which the heating of the volume between the inner and outervessels is achieved by hot gases produced in the process.
 4. Anapparatus as claimed in claim 3, in which the fuel material isindirectly heated to approximately 850° C. in an oxygen free atmosphere.5. An apparatus as claimed in claim 4 in which the heating medium issupplied by the product of the process.
 6. An apparatus as claimed inany one of claims 3 to 5 in which the internal rotary vessel is providedwith internal longitudinal substantially parallel vanes.
 7. An apparatusas claimed in claim 6 in which the said vanes are of a profile torestrict the fall of the material whilst providing an increased heatedsurface area.
 8. An apparatus as claimed in claim 7 in which the vanesare substantially L shaped in cross section.
 9. An apparatus as claimedin claim 8 in which the vanes are attached by means of bolts andinterlocking mortises to allow for expansion and contraction whilstholding the vanes firmly in place.
 10. An apparatus as claimed in anyone of claims 1 to 9 in which the outer vessel is insulated with a heatretentive material.
 11. An apparatus as claimed in any one of theprevious claims including a gas quench system.
 12. An apparatus asclaimed in any one of the previous claims including a secondarygasifier.
 13. An apparatus and process as claimed in claim 12 in whichthe secondary gasifier comprises two substantially concentric dynamicspirals induced by tangential introduction of fuel into a refractorylined vessel.
 14. An apparatus as claimed in claim 13 in which thesecondary gasifier is fuelled by char, oils and/or tar
 15. An apparatusas claimed in claim 14 in which the secondary gasifier is fuelledprincipally by the product of the primary gasifier.
 16. An apparatus asclaimed in any of claims 12 to 14 7 in which the exhaust gas from thesecondary gasifier is ducted to heat the primary gasifier which in turnprovides a fuel source for the secondary gasifier.
 17. An apparatus asclaimed in claim 16 in which residual ash is ejected at the base of thesecondary gasifier allowing cleaned gases to be extracted by thecyclonic flow.
 18. An apparatus as claimed in any of claims 13 to 17 inwhich steam is injected into the secondary gasifier with the fuel. 19.An apparatus as claimed in any of claims 13 to 18 in which contaminatedliquors are injected into the secondary gasifier for cleaning andpurification.
 20. An apparatus and process for the gasification of wasteresidues or any other fuel of significant calorific value constructed oradapted to operate substantially as described herein with reference toand as illustrated in the drawing.